1. Field of the Invention
The present invention relates to a detection device for detecting an electromagnetic wave of a generally wide frequency region including the infrared region and ranging between a longer wavelength region of the millimeter wave region to the terahertz bandwidth (30 GHz to 30 THz) and a shorter wavelength region of the visible light region, and to an image forming device using the detection device.
2. Description of the Related Art
As a detection device in the infrared region, there have hitherto been widely known a thermal detection device having no wavelength dependency and a quantum detection device having wavelength dependency. Most of these devices utilize a feature of infrared detection. For example, these are applied in a wide range of fields and various applications such as a human body detector at an automatic door, a temperature sensor in a fire alarm or air conditioner, a gas detector using an infrared absorption spectrum of gas, and space research in the submillimeter wave/far-infrared region. Here, the thermal detection device includes, for example, a pyroelectric device (LiTaO3, TGS, etc.), a Golay cell and a bolometer. In addition, the quantum detection device includes, for example, an intrinsic semiconductor device (InGaAs PIN photodiode, MCT photoconduction device, etc.) and an impurity semiconductor device. In addition, a schottky barrier diode can perform the both operations.
Comparing the both types of devices with each other, the thermal detection device has a feature of simple, convenient usability, and for example, a typical thermal detection device does not need cooling, while a typical quantum detection device needs cooling. On the other hand, it is known that the thermal detection device generally has small detectivity because of its small photoreceptive sensitivity and large NEP (Noise Equivalent Power). The detectivity can be represented by specific detectivity D*. Generally, it is said that the larger D* (D-Star) is, the better a detection device is, and in addition, in order to easily compare different devices with each other, D* is given as a reciprocal of NEP normalized to square root of unit area. Comparing the both types of devices with each other in terms of the specific detectivity, for example, the specific detectivity D* of a typical quantum detection device in the mid-infrared region is within the range from 1010 cm·Hz1/2/w to 1011 cm·Hz1/2/w, and the D* of a typical thermal detection device is within the range from 108 cm·Hz1/2/w to 109 cm·Hz1/2/w. Therefore, as seen from the D*, it can be said that the thermal detection device is inferior in performance to the quantum detection device by about two orders of magnitude.
As described above, the thermal detection device has convenient usability, but on the one hand, it has small detectivity and a very wide, detectable frequency region. Then, in order to improve detectivity in a particular wavelength region, a method is usually used in which the noise on the whole of a device is reduced by using a wavelength selection filter that allows an electromagnetic wave in a part of a detectable frequency region to pass therethrough. For example, Japanese Patent Application Laid-Open No. H08-145787 discloses a method of narrowing down infrared light to be detected by a pyroelectric device by use of a diffraction optical lens. At this time, the diffraction optical lens functions as a wavelength selection filter for infrared light. Furthermore, since the utilization of the diffraction optical lens reduces lowering of the light intensity of infrared light due to passing through the filter, the device disclosed in the patent document has a structure such that the photoreceptive sensitivity of the device as a whole is difficult to lower.
In addition, on the other hand, the development of the light detection technology has enabled detection of an optical electric field involved with surface plasmon. Tsutomu Ishii et al.; Jpn. Jour. Appl. Phys., Vol. 44 (2005), L364 disclose a structure in which photodiodes are integrated on a metal diffraction ring for selectively enhancing optical electric field of a particular wavelength which has a hole with a diameter not larger than the wavelength provided at the center thereof and has some grooves provided at intervals of a dimension similar to the wavelength. At this time, light desired to be detected is concentrated in the form of surface plasmon at the center of the metal diffraction ring to show an effect to enhance the optical electric field at the central hole where the photodiodes are positioned. Therefore, there is a possibility that utilization of such enhancing effect may improve the photoreceptive sensitivity of the device as a whole.